Organic Salts For High Voltage Organic And Transparent Solar Cells

ABSTRACT

Photo-active devices including a substrate, a first electrode, an active layer including an organic salt or salt mixture that selectively or predominantly harvests light from the near infrared or infrared regions of the solar spectrum, and a second electrode. The devices are either visibly transparent or visibly opaque and can be utilized in single- or multi-junction devices.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.15/791,949 filed on Oct. 24, 2017, which is a continuation applicationof PCT International Patent Application Serial No. PCT/US2016/026169filed on Apr. 6, 2016, which claims the benefit of U.S. ProvisionalApplication No. 62/153,299, filed on Apr. 27, 2015, which are herebyincorporated by reference herein.

GOVERNMENT RIGHTS

This invention was made with government support under CBET1254662awarded by the National Science Foundation. The government has certainrights in the invention.

FIELD

The present disclosure relates to organic small molecule salts that canharvest extended near-infrared light for efficient organic, transparent,and multijunction photovoltaic and photodetector devices.

BACKGROUND

This section provides background information related to the presentdisclosure which is not necessarily prior art.

Organic Photovoltaics (OPVs) are rapidly approaching commercialviability because of their potential for inexpensive, high-throughputmanufacturing and unique applications, such as flexible and transparentsolar cells. Compared to conventional inorganic cells, however, OPVs aretypically limited by spectral overlap with the sun. Few molecules havebeen demonstrated with efficient photoconversion past 900 nm, leavingalmost half the incident solar photon flux unutilized. Molecules withabsorption in this region often suffer from low open-circuit voltages(VOCs). For example, SnPc and PbPc, which have absorption cut-offs near950 nm and 1000 nm, exhibit VOCs of 0.42 V and 0.47 V, respectively,nearly half of the realistic excitonic voltage limit. Because of thisvoltage limitation and limited spectral coverage, current demonstrationsof Transparent Organic Photovoltaics (TPVs) with high transparency havebeen limited to 2-4%. Expanding the catalog of efficient NIR moleculeswould help advance the performance of panchromatic tandem cells andsingle- and multi-junction transparent photovoltaics.

Polymethines are one of the most promising classes of molecules tosatisfy the need for efficient, NIR-harvesting and visibly transparentNIR-harvesting compounds. Polymethines are a class of ionic organicsalts that have gained attention for photovoltaic applications due totheir tunable absorption, high extinction coefficients, and highsolubility. Recently, polymethines with absorption maxima in thenear-infrared have been employed in OPV devices with efficiencies of1.5-2.8% for opaque devices, 0.9-2.2% for semitransparent devices, andtransparent luminescent solar concentrators.

Recent efforts to improve cyanine photovoltaics are focused on studyingthe influence of the cation chemistry and counterions of larger gapmolecules on device performance, optical properties, and solid statepacking. For instance, PF₆ ⁻ exhibits higher fill factors than ClO₄ ⁻ inbilayer devices. Also, exchanging PF₆ ⁻ for Δ-TRISPHAT, a bulky, chiralanion with fluorinated phenyl groups, Voc and Jsc could be enhanced bynearly 50% in larger gap systems. The selection of Δ-TRISPHAT as asuitable counterion was made based upon a photophysical and structuralstudy which showed that the Δ-TRISPHAT counterion reduced polarizationof the cyanine cation in the solid state and exhibited near zero bondlength alternation (BLA). Harder counterions such as Br⁻, I⁻, and PF₆ ⁻showed BLAs of 2-6 pm. Nonetheless, there remains a need to develop neworganic small molecule salts that can harvest extended near-infraredlight for efficient organic, transparent, and multijunction photovoltaicdevices and detectors.

SUMMARY

This section provides a general summary of the disclosure, and is not acomprehensive disclosure of its full scope or all of its features.

The present technology provides a photo-active device that includes asubstrate, a first electrode deposited within the substrate or on atleast one surface of the substrate, an active layer comprising anorganic salt and optionally a complimentary electron donor or electronacceptor, and a second electrode. The organic salt selectively absorbslight in the near infrared or infrared regions of the solar spectrum. Invarious embodiments, the organic salt includes a polymethine derivativeorganic cation.

The present technology also provides a photo-active device that has anactive layer having an organic salt. The organic salt includes acounterion, wherein the counterion is selected from halides, arylborates, carboranes,(Λ,R)-(1,1′-binaphthalene-2,2′diolato)(bis(tetrachlor-1,2-benzenediolato)phosphate(V))(BINPHAT), [Δ-tris(tetrachloro-1,2-benzenediolato)phosphate(V)](TRISPHAT), fluoroantimonates; fluorophosphates, fluoroborates,derivatives thereof, and combinations thereof. The organic salt alsoincludes a cation, such as a polymethine cation or cyanine cation.

Additionally, the current technology provides a photo-active device thatincludes an active layer having a polymethine salt. The polymethine saltincludes a cation and a counterion. In various embodiments, thephoto-active device is a visibly transparent or visibly opaquephotovoltaic or a visibly transparent or visibly opaque photodetector.

Further, the present technology provides a method of manufacturing aphoto-active device. The method includes blending together at least twoorganic salts to generate an anion alloy, wherein the organic salts havethe same cation, but different anions, and disposing the anionalloy-cation pair between a first electrode and a second electrode. Invarious embodiments, the at least two organic salts have differentanions individually selected from the group consisting of F⁻, Cl⁻, I⁻,and Br⁻; aryl borates, such as tetraphenylborate, tetra(p-tolyl)borate,tetrakis(4-biphenylyl)borate, tetrakis(1-imidazolyl)borate,tetrakis(2-thienyl)borate, tetrakis(4-chlorophenyl)borate,tetrakis(4-fluorophenyl)borate, tetrakis(4-tert-butylphenyl)borate,tetrakis(pentafluorophenyl)borate (TPFB),tetrakis[3,5-bis(trifluoromethyl)phenyl]borate (TFMPB),[4-[bis(2,4,6-trimethylphenyl)phosphino]-2,3,5,6-tetrafluorophenyl]hydrobis(2,3,4,5,6-pentafluorophenyl)borate,[4-di-tert-butylphosphino-2,3,5,6-tetrafluorophenyl]hydrobis(2,3,4,5,6-pentafluorophenyl)borate;carboranes,(Λ,R)-(1,1′-binaphthalene-2,2′diolato)(bis(tetrachlor-1,2-benzenediolato)phosphate(V))(BINPHAT), [Δ-tris(tetrachloro-1,2-benzenediolato)phosphate(V)](TRISPHAT); fluoroantimonates, such as hexafluoroantimonate;fluorophosphates, such as hexafluorophosphate; fluoroborates, such astetrafluoroborate (BF₄ ⁻); and derivatives thereof.

Further areas of applicability will become apparent from the descriptionprovided herein. The description and specific examples in this summaryare intended for purposes of illustration only and are not intended tolimit the scope of the present disclosure.

DRAWINGS

The drawings described herein are for illustrative purposes only ofselected embodiments and not all possible implementations, and are notintended to limit the scope of the present disclosure.

FIG. 1A is a schematic illustration of a first device according to thepresent technology;

FIG. 1B is a schematic illustration of a second device according to thepresent technology;

FIG. 2A shows a molecular structure of a polymethine cation (Cy⁺),wherein the blue atoms are nitrogen and the green atom is chlorine;

FIG. 2B shows molecular structures of counterions according to thepresent technology: (1) I⁻, (2) PF₆ ⁻, (3) SbF₆ ⁻, (4) Δ-TRISPHAT(TRIS), and (5) Tetrakis(pentafluorophenyl)borate (TPFB);

FIG. 2C is an illustration of an exemplary structure of a transparent oropaque solar cell;

FIG. 3A shows a J-V curve for representative devices for each counterionshown in FIG. 1B under 1 sun illumination, wherein an optimized opaquedevice structure for each counterion was ITO (120 nm)/MoO3 (10 nm)/CyX(8-12 nm)/C60 (40 nm)/BCP (7.5 nm)/Ag (100 nm);

FIG. 3B shows a J-V curve for representative devices for each counterionshown in FIG. 1B under dark conditions, wherein an optimized opaquedevice structure for each counterion was ITO (120 nm)/MoO3 (10 nm)/CyX(8-12 nm)/C60 (40 nm)/BCP (7.5 nm)/Ag (100 nm);

FIG. 4A is a graph showing normalized extinction coefficients of variousCy⁺ salts

FIG. 4B is a graph showing external quantum efficiency of partiallyoptimized devices structures for each Cy⁺ salt shown in FIG. 4A;

FIG. 5A shows transmission scans of full transparent devices for Cy(4)and Cy(5) of FIGS. 2A and 2B;

FIG. 5B is a photograph of a transparent solar cell device according tothe present technology placed over a Michigan State University Spartanhelmet logo;

FIG. 6A is a graph of responsivity versus intensity for each Cy+ saltshown in FIG. 6D, wherein responsivities are comparable across a rangeof counterions;

FIG. 6B is a graph of open-circuit voltages (VOCs) versus intensity foreach Cy+ salt shown in FIG. 6D, wherein the VOCs for each salt isdramatically different;

FIG. 6C is a graph of power conversion efficiency (PCE) versus intensityfor each Cy⁺ salt shown in FIG. 6D;

FIG. 6D is a graph of fill factor (FF) versus intensity for each Cy⁺salt, wherein the FF is considerably lower for Cyl, but recovered at lowintensities;

FIG. 7A is a graph showing thickness dependence of external quantumefficiency (EQE) for CyTRIS;

FIG. 7B is a graph showing thickness dependence of EQE for CyTPFB;

FIG. 7C is a graph showing thickness dependence of EQE for CyPF₆;

FIG. 7D is a graph showing thickness dependence of EQE for Cyl;

FIG. 8 is a graph showing thickness dependence of peak EQE in the NIRfor Cy⁺ with various counterions, wherein Cy⁺TRIS⁻ and Cy⁺TPFB⁻ peak ata slightly higher thickness and higher efficiency than the othercounterions;

FIG. 9A is a graph showing normalized transmission spectra for fourorganic salt films with optical activity deeper in the near infrared(NIR);

FIG. 9B is a graph of EQE spectra for each of the deeper NIR activeorganic salts in devices from FIG. 9A;

FIG. 10 shows molecular structures of additional exemplary anions thathave been identified as suitable counterion pairings to achieve highvoltages with NIR organic salts;

FIG. 11 shows a range of exemplary selectively harvesting NIR organiccations that have been identified as suitable for devices andtransparent cells;

FIG. 12A shows absorption spectra for three cells with complimentaryabsorption for multijunction incorporation;

FIG. 12B is a schematic illustration of multijunction opaque ortransparent cells with complimentary NIR absorbers in each sub-cell forenhanced performance and efficiency;

FIG. 13 is a graph showing VOCs versus excitonic bandgap;

FIG. 14A is a graph showing J-V data for a device with2-[2-[2-chloro-3-[2-(1,3-dihydro-3,3-dimethyl-1-ethyl-2H-benz[e]indol-2-ylidene)ethylidene]-1-cylohexen-1-yl]-ethenyl]-3,3-dimethyl-1-ethyl-1H-benz[e]indolium(Cy) tetrakis(pentafluorophenyl)borate (TPFB) and Cytetrakis[3,5-bis(trifluoromethyl)phenyl]borate (TFMPB);

FIG. 14B is a graph showing EQE data for a device with CyTPFB andCyTFMPB;

FIG. 15A is a graph showing Voc as a function of mole percent CyTPFB inblends with CyPF₆;

FIG. 15B is a schematic representation of an enhanced energy levelalignment between CyTPFB and C₆₀, as well as the tunability of aninterface gap via mixtures of CyPF₆ and CyTPFB, wherein the insetillustrates the mechanism of density of states (DOS) averaging forshifts in frontier energy levels and enhanced Voc;

FIG. 16A shows structures of heptamethine salt cations (1(1-Butyl-2-(2-[3-[2-(1-butyl-1H-benzo[cd]indol-2-ylidene)-ethylidene]-2-diphenylamino-cyclopent-1-enyl]-vinyl)-benzo[cd]indolium)and 2(1-Butyl-2-(2-[3-[2-(1-butyl-1H-benzo[cd]indol-2-ylidene)-ethylidene]-2-phenyl-cyclopent-1-enyl]-vinyl)-benzo[cd]indolium)and anions (BF₄ ⁻ and tetraphenylfluoroborate, TPFB⁻);

FIG. 16B is a graph showing normalized thin film absorption(1-Transmission) of salts provided by the structures shown in FIG. 14A;

FIG. 16C is a summary of high-resolution mass spectrometry m/z spectrafor cations and anions shown in FIG. 16A, wherein multiple peaks andtheir relative heights represent the isotopic abundances of thecompound's constituent elements;

FIG. 17A is a graphic illustration of a device architecture used forphotovoltaic and photodetector structures;

FIG. 17B is a current density-voltage (J-V) graph;

FIG. 17C shows external quantum efficiency (EQE) curves for devices withsalts with thicknesses of 12 nm (inset highlights NIR EQE);

FIG. 18A is an energy schematic that illustrates the deepening of thelowest unoccupied molecular orbital (LUMO) level and increase ininterface gap (I_(G)) after counterion exchange from a small anion BF₄⁻to a bulky and weakly-coordinating anion TPFB⁻;

FIG. 18B is a schematic D-A band structure as a function of donorthickness (t_(D));

FIG. 19A is a graph showing Voc (left axis) and EQE (right axis) asfunctions of mole fraction TPFB for blends of (black) 1-BF₄ and 1-TPFBand (blue) 2-BF₄ and 2-TPFB at t_(D)=6 nm;

FIG. 19B is a graph showing a wavelength of EQE shown in inset: 1200 nmfor 1-BF₄ and 1-TPFB and 1350 nm for 2-BF₄ and 2-TPFB; and

FIG. 20 shows specific detectivity D* spectra for each salt donor.

Corresponding reference numerals indicate corresponding parts throughoutthe several views of the drawings.

DETAILED DESCRIPTION

Example embodiments will now be described more fully with reference tothe accompanying drawings.

The terminology used herein is for the purpose of describing particularexample embodiments only and is not intended to be limiting. As usedherein, the singular forms “a,” “an,” and “the” may be intended toinclude the plural forms as well, unless the context clearly indicatesotherwise. The terms “comprises,” “comprising,” “including,” and“having,” are inclusive and therefore specify the presence of statedfeatures, elements, compositions, steps, integers, operations, and/orcomponents, but do not preclude the presence or addition of one or moreother features, integers, steps, operations, elements, components,and/or groups thereof. Although the open-ended term “comprising,” is tobe understood as a non-restrictive term used to describe and claimvarious embodiments set forth herein, in certain aspects, the term mayalternatively be understood to instead be a more limiting andrestrictive term, such as “consisting of” or “consisting essentiallyof.” Thus, for any given embodiment reciting compositions, materials,components, elements, features, integers, operations, and/or processsteps, the present disclosure also specifically includes embodimentsconsisting of, or consisting essentially of, such recited compositions,materials, components, elements, features, integers, operations, and/orprocess steps. In the case of “consisting of,” the alternativeembodiment excludes any additional compositions, materials, components,elements, features, integers, operations, and/or process steps, while inthe case of “consisting essentially of,” any additional compositions,materials, components, elements, features, integers, operations, and/orprocess steps that materially affect the basic and novel characteristicsare excluded from such an embodiment, but any compositions, materials,components, elements, features, integers, operations, and/or processsteps that do not materially affect the basic and novel characteristicscan be included in the embodiment.

Although the terms first, second, third, etc. may be used herein todescribe various steps, elements, components, regions, layers and/orsections, these steps, elements, components, regions, layers and/orsections should not be limited by these terms, unless otherwiseindicated. These terms may be only used to distinguish one step,element, component, region, layer or section from another step, element,component, region, layer or section. Terms such as “first,” “second,”and other numerical terms when used herein do not imply a sequence ororder unless clearly indicated by the context. Thus, a first step,element, component, region, layer or section discussed below could betermed a second step, element, component, region, layer or sectionwithout departing from the teachings of the example embodiments.

Throughout this disclosure, the numerical values represent approximatemeasures or limits to ranges to encompass minor deviations from thegiven values and embodiments having about the value mentioned as well asthose having exactly the value mentioned. Other than in the workingexamples provided at the end of the detailed description, all numericalvalues of parameters (e.g., of quantities or conditions) in thisspecification, including the appended claims, are to be understood asbeing modified in all instances by the term “about” whether or not“about” actually appears before the numerical value. “About” indicatesthat the stated numerical value allows some slight imprecision (withsome approach to exactness in the value; approximately or reasonablyclose to the value; nearly). If the imprecision provided by “about” isnot otherwise understood in the art with this ordinary meaning, then“about” as used herein indicates at least variations that may arise fromordinary methods of measuring and using such parameters.

As referred to herein, ranges are, unless specified otherwise, inclusiveof endpoints and include disclosure of all distinct values and furtherdivided ranges within the entire range. Thus, for example, a range of“from A to B” or “from about A to about B” is inclusive of A and of B.Disclosure of values and ranges of values for specific parameters (suchas temperatures, molecular weights, weight percentages, etc.) are notexclusive of other values and ranges of values useful herein. It isenvisioned that two or more specific exemplified values for a givenparameter may define endpoints for a range of values that may be claimedfor the parameter. For example, if Parameter X is exemplified herein tohave value A and also exemplified to have value Z, it is envisioned thatParameter X may have a range of values from about A to about Z.Similarly, it is envisioned that disclosure of two or more ranges ofvalues for a parameter (whether such ranges are nested, overlapping ordistinct) subsume all possible combination of ranges for the value thatmight be claimed using endpoints of the disclosed ranges. For example,if Parameter X is exemplified herein to have values in the range of1-10, or 2-9, or 3-8, it is also envisioned that Parameter X may haveother ranges of values including 1-9, 1-8, 1-3, 1-2, 2-10, 2-8, 2-3,3-10, and 3-9.

The current technology provides apparatuses and methods directed tophoto-active devices and light harvesting systems, such as photovoltaicsand photodetectors. The photo-active devices and light harvestingsystems can be opaque, transparent, heterojunction cells, ormulti-junction cells. The devices and systems include organic salts thatselectively or predominately harvest light with wavelengths in theinfrared (IR) region of the solar spectrum, near IR (NIR) region of thesolar spectrum, or both the IR and NIR regions of the solar spectrum.

With reference to FIG. 1A, the present technology provides aphoto-active device 10. The photo-active device 10 comprises a substrate12, a first electrode 14, an active layer 16 comprising an organic saltand optionally a complimentary electron donor or electron acceptor, anda second electrode 18. In embodiments where the active layer 16 does notinclude a complimentary electron donor or electron acceptor, thecomplimentary electron donor or electron acceptor may be provided in aseparate distinct layer (not shown). In various embodiments, thephoto-active device 10 includes at least one, or a plurality of, activelayers 16, at least one, or a plurality of, complimentary layers thatinclude electron donors or electron acceptors, or at least one of, or aplurality of, both active layers 16 and complimentary layers. The activelayer 16 and any complimentary layers have a thickness of from about 1nm to about 300 nm, or from about 3 nm to about 100 nm. Although notshown, in some embodiments the photo-active device 10 also includesbuffer layers positioned between any of the layers and electrodes 12,14, 16, 18, which may block excitons, modify a work function orcollection barrier, induce ordering or templating, or serve as opticalspacers. The photo-active device 10 has an open circuit voltage that iswithin about 30% or about 20% of the excitonic limit as defined in Luntet al., “Practical Roadmap and Limits to Nanostructured Photovoltaics”(Perspective) Adv. Mat. 23, 5712-5727, 2011, which is incorporatedherein by reference. Briefly, the form for the excitonic limiting opencircuit voltage, i.e., the excitonic limit, under 1 Sun follows roughly80% of the theoretical Shockley-Queisser thermodynamically limited opencircuit voltage (See FIG. 13) that is limited by the smallest of theband gaps. The factor of 80% in the excitonic limit accounts for theminimum energetic driving force required to dissociate excitons.Alternatively, the photo-active device 10 has an open circuit voltagethat is within about 50% or about 35% of the thermodynamic limit.

The substrate 12 of the photo-active device 10 can be any visiblytransparent or visibly opaque material 12 known in the art. Non-limitingexamples of transparent substrates include glass, low iron glass,plastic, poly(methyl methacrylate) (PMMA), poly-(ethylmethacrylate)(PEMA), (poly)-butyl methacrylate-co-methyl methacrylate (PBMMA),polyethylene terephthalate (PET), and polyimides, such as Kapton®polyimide films (DuPont, Wilmington, Del.). Non-limiting examples ofopaque substrates include amorphous silicon, crystalline silicon, halideperovskites, stainless steel, metals, metal foils, and gallium arsenide.

The substrate 12 comprises the first electrode 14. As shown in FIG. 1A,the first electrode 14 is positioned or deposited on a first surface ofthe substrate 12 as, for example, a thin film, by solution deposition,drop casting, spin-coating, doctor blading, vacuum deposition, plasmasputtering, or e-beam deposition, as non-limiting examples, withthicknesses that allow for active-layer films that are visiblytransparent or visibly opaque. However, in various embodiments, multipleelectrodes 14 may be present, such as with a device having a firstelectrode on a first surface of a substrate and on a second opposingsurface of the substrate (not shown). In another embodiment, depicted asFIG. 1B, a photo-active device 10′ has the same components as thephoto-active device 10 of FIG. 1A (a substrate 12, an electrode 14, andan active layer 16, and optionally buffer layers); however, the firstelectrode 14 is positioned within the substrate 12. Therefore, thesubstrate 12 may include materials that act as the electrode 14, suchthat the substrate 12 and electrode 14 are visibly indistinguishable. Inany embodiment, the first electrode 14 can be composed of any materialknown in the art. Non-limiting examples of electrode materials includeindium tin oxide (ITO), aluminum doped zinc oxide (AZO), indium zincoxide, zinc oxide, and gallium zinc oxide (GZO), ultra-thin metals, suchas Ag, Au, and Al, graphene, graphene oxide,poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS), andcombinations thereof. In various embodiments, the first electrode 14 hasa thickness of from about 1 nm to about 500 nm, from about 1 nm to about200 nm, from about 10 nm to about 200 nm, from about 15 nm to about 150nm, or from about 500 nm or less. Notwithstanding, it is understood thatchanging the thickness of the first electrode 14 may alter the visibletransparency of the photo-active device 10 via modulation of complexinterference associated with the multiple layers 12, 14, 16 in thephoto-active device 10.

The active layer 16 is positioned or disposed on a surface of theelectrode 14 in the photo-active device 10, such as by solutiondeposition, drop casting, spin-coating, doctor blading, or vacuumdeposition, as non-limiting examples, with thicknesses that allow forfilms that are visibly transparent or visibly opaque. Therefore, thephoto-active device 10 includes the first electrode 14, which has afirst surface in contact with the substrate 12 and a second surface indirect contact with active layer 16. However, in some embodiments, abuffer layer is positioned between the substrate 12 and the firstelectrode 14 and/or a buffer layer is positioned between the firstelectrode 14 and the active layer 16. Also, the second electrode 18 maybe in direct contact with the active layer 16 or a buffer layer may bepositioned between the second electrode 18 and the active layer 16. Insome embodiments, such as with the photo-active device 10′ of FIG. 1B,the first electrode 14 is positioned within the substrate 12. In suchembodiments, the active layer 16 is positioned on, and as in directcontact with, a first surface of the substrate 12.

As mentioned above, the active layer 16 comprises an organic salt and acomplimentary electron donor or electron acceptor. The photoactiveorganic salt can, as non-limiting examples, be comprised of anycombination of cations shown in FIG. 10 and anions shown in FIG. 11 withselected absorption profiles shown, for example, in FIGS. 9A and 9B.However, it is understood that any combination of cation or aniondescribed herein can be used in the photo-active device 10. In variousembodiments, the organic salts act as electron donors and are pairedwith electron acceptors, such as C₆₀ in the active layers 16. In otherembodiments, the organic salts act as electron acceptors and are pairedwith electron donors or other organic molecules or salts. Here, theorganic salts harvest light with wavelengths in the NIR, or IR regionsof the solar spectrum, or both the NIR and IR regions. As used herein,“VIS” light has a wavelength from about 400 nm to about 675 nm, “NIR”light has a wavelength from about 675 nm to about 1500 nm and “IR” lighthas a wavelength form about 1500 nm to about 1 mm. Also, as used hereinthe terms “transparent” or “visibly transparent” refer to devices thathave an average visible transparency, weighted by the photopic responseof an eye, of about 50% or more for specular transmission. The terms“opaque” or “visibly opaque” refer to devices that have an averagevisible transparency, weighted by the photopic response of an eye of 10%or less for specular transmission. Devices that have an average visibletransparency, weighted by the photopic response of an eye of between 10%to 50% for specular transmission are “semitransparent.” As a person ofordinary skill in the art appreciates, organic salts include a cationand a counterion, i.e., an anion. In various embodiments, the organicsalt is a polymethine salt or cyanine salt that selectively orpredominately harvests NIR and/or IR light, such as the exemplarystructures shown in FIG. 11. Non-limiting examples of suitable organiccations (which are “base cations” relative to their derivatives) thatform organic salts in the presence of a counterion include1-Butyl-2-(2-[3-[2-(1-butyl-1H-benzo[cd]indol-2-ylidene)-ethylidene]-2-phenyl-cyclopent-1-enyl]-vinyl)-benzo[cd]indolium(peak absorbance at 1024 nm),1-Butyl-2-(2-[3-[2-(1-butyl-1H-benzo[cd]indol-2-ylidene)-ethylidene]-2-chloro-cyclohex-1-enyl]-vinyl)-benzo[cd]indolium(peak absorbance at 1014 nm),1-Butyl-2-(2-[3-[2-(1-butyl-1H-benzo[cd]indol-2-ylidene)-ethylidene]-2-phenyl-cyclohex-1-enyl]-vinyl)-benzo[cd]indolium(peak absorbance at 997 nm),1-Butyl-2-(2-[3-[2-(1-butyl-1H-benzo[cd]indol-2-ylidene)-ethylidene]-2-diphenylamino-cyclopent-1-enyl]-vinyl)-benzo[cd]indolium(peak absorbance at 996 nm),1-Butyl-2-[7-(1-butyl-1H-benzo[cd]indol-2-ylidene)-hepta-1,3,5-trienyl]-benzo[cd]indolium(peak absorbance at 973 nm),2-[2-[2-chloro-3-[2-(1,3-dihydro-3,3-dimethyl-1-ethyl-2H-benz[e]indol-2-ylidene)ethylidene]-1-cylohexen-1-yl]-ethenyl]-3,3-dimethyl-1-ethyl-1H-benz[e]indolium(“Cy⁻”; peak absorbance at 820 nm),N,N,N′,N′-Tetrakis-(p-di-n-butylaminophenyl)-p-benzochinon-bis-immonium(peak absorbance at 1065 nm),4-[2-[2-Chloro-3-[(2,6-diphenyl-4H-thiopyran-4-ylidene)ethylidene]-1-cyclohexen-1-yl]ethenyl]-2,6-diphenylthiopyrylium,1-Butyl-2-[2-[3-[(1-butyl-6-chlorobenz[cd]indol-2(1H)-ylidene)ethylidene]-2-chloro-5-methyl-1-cyclohexen-1-yl]ethenyl]-6-chlorobenz[cd]indolium,1-Butyl-2-[2-[3-[(1-butyl-6-chlorobenz[cd]indol-2(1H)-ylidene)ethylidene]-2-chloro-1-cyclohexen-1-yl]ethenyl]-6-chlorobenz[cd]indolium,Dimethyl{4-[1,7,7-tris(4-dimethylaminophenyl)-2,4,6-heptatrienylidene]-2,5-cyclohexadien-1-ylidene}ammonium,5,5′-Dichloro-11-diphenylamino-3,3′-diethyl-10,12-ethylenethiatricarbocyamine,2-[2-[2-Chloro-3-[2-(1,3-dihydro-1,1,3-trimethyl-2H-benzo[e]-indol-2-ylidene)-ethylidene]-1-cyclohexen-1-yl]-ethenyl]-1,1,3-trimethyl-1H-benzo[e]indolium,2-[2-[2-Chloro-3-[2-(1,3-dihydro-1,3,3-trimethyl-2H-indol-2-ylidene)-ethylidene]-1-cyclopenten-1-yl]-ethenyl]-1,3,3-trimethyl-3H-indolium,2-[2-[3-[(1,3-Dihydro-3,3-dimethyl-1-propyl-2H-indol-2-ylidene)ethylidene]-2-(phenylthio)-1-cyclohexen-1-yl]ethenyl]-3,3-dimethyl-1-propylindolium,1,1′,3,3,3′,3′-4,4′,5,5′-di-benzo-2,2′-indotricarbocyanine perchlorate,2-[2-[2-Chloro-3-[2-(1,3-dihydro-1,3,3-trimethyl-2H-indol-2-ylidene)-ethylidene]-1-cyclohexen-1-yl]-ethenyl]-1,3,3-trimethyl-3H-indolium,3,3′-Diethylthiatricarbocyanine,2-[[2-[2-[4-(dimethylamino)phenyl]ethenyl]-6-methyl-4H-pyran-4-ylidene]methyl]-3-ethyl,2-[7-(1,3-Dihydro-1,3,3-trimethyl-2H-indol-2-ylidene)-1,3,5-heptatrienyl]-1,3,3-trimethyl-3H-indolium,derivatives thereof, and combinations thereof. As used herein,“derivatives” of the organic cations refer to or include organic cationsthat resemble a base organic cation, but that contain minor changes,variations, or substitutions, such as in, for example, solubilizinggroups (e.g., R, R′, R″, S, S′, S″ in FIG. 11) with varying alkyl chainlength or substitution with other solubilizing groups, which do notsubstantially change the bandgap or electronic properties as shown inFIG. 10 as well as substitutions at a central methane position (X,Y)with various halides or ligands. Non-limiting examples of counterions(which are “base counterions” relative to their derivatives) or anionsthat form salts with the organic cations include halides, such as F⁻,Cl⁻, I⁻, and Br⁻; aryl borates, such as tetraphenylborate,tetra(p-tolyl)borate, tetrakis(4-biphenylyl)borate,tetrakis(1-imidazolyl)borate, tetrakis(2-thienyl)borate,tetrakis(4-chlorophenyl)borate, tetrakis(4-fluorophenyl)borate,tetrakis(4-tert-butylphenyl)borate, tetrakis(pentafluorophenyl)borate(TPFB), tetrakis[3,5-bis(trifluoromethyl)phenyl]borate (TFMPB),[4-[bis(2,4,6-trimethylphenyl)phosphino]-2,3,5,6-tetrafluorophenyl]hydrobis(2,3,4,5,6-pentafluorophenyl)borate,[4-di-tert-butylphosphino-2,3,5,6-tetrafluorophenyl]hydrobis(2,3,4,5,6-pentafluorophenyl)borate;carboranes,(Λ,R)-(1,1′-binaphthalene-2,2′diolato)(bis(tetrachlor-1,2-benzenediolato)phosphate(V))(BINPHAT), [Δ-tris(tetrachloro-1,2-benzenediolato)phosphate(V)](TRISPHAT); fluoroantimonates, such as hexafluoroantimonate (SbF₆ ⁻);fluorophosphates, such as hexafluorophophosphate (PF₆ ⁻); fluoroborates,such as tetrafluoroborate (BF₄); derivatives thereof; and combinationsthereof. As used herein, “derivatives” of the counterion refer to orinclude counterions or anions that resemble a base counterion, but thatcontains minor changes, variations, or substitutions, that do notsubstantially change the ability of the counterion to form a salt withthe organic cations.

As shown in FIG. 1A, the second electrode 18 is positioned or depositedon a surface of the active layer 16 as, for example, a thin film. Thesecond electrode 18, is positioned or deposited on the surface of theactive layer 16 by, solution deposition, drop casting, spin-coating,doctor blading, vacuum deposition, plasma sputtering, or e-beamdeposition, as non-limiting examples, with thicknesses that allow foractive-layer films that are visibly transparent or visibly opaque.Therefore, the second electrode 18 is in contact with a surface of theactive layer 16 that opposes a surface of the active layer that is incontact with the first electrode 14. The second electrode 18 can becomposed of any material known in the art. Non-limiting examples ofelectrode materials include indium tin oxide (ITO), aluminum doped zincoxide (AZO), zinc oxide, and gallium zinc oxide (GZO), ultra-thinmetals, such as Ag, Au, and Al, graphene, graphene oxide,poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS), andcombinations thereof. In various embodiments, the second electrode 18has a thickness of from about 1 nm to about 500 nm, from about 1 nm toabout 200 nm, from about 10 nm to about 200 nm, from about 15 nm toabout 150 nm, or from about 500 nm or less. Notwithstanding it isunderstood that changing the thickness of the second electrode 18 mayalter the visible transparency of the photo-active device via modulationof complex optical interference and absorption associated with themultiple layers 12, 14, 16 in the photo-active device 10.

With further regard to the first electrode 14 and the second electrode18, at least one of the electrodes 14, 18 is visibly transparent inembodiments where the device is visibly opaque. In embodiments where thedevice is visibly transparent, both the first electrode 14 and thesecond electrode 18 are visibly transparent with thicknesses tailored tooptimize the visible transparency in the active layer 16.

Although not shown in FIG. 1A or 1B, in various embodiments thephoto-active devices 10, 10′ further include additional active layers,such as electron donors and/or electron acceptors, electrode layers, orcombinations thereof. For example, additional active layers may includemolybdenum oxide (MoO₃), bathocuproine (BCP), C₆₀, or ITO. Additionalelectrodes may be composed of layers of Ag, Au, Pt, Al, or Cu.Additional non-limiting examples of electron acceptors include of C₇₀,C₈₄, [6,6]-phenyl-C61-butyric acid methyl ester, TiO₂, metal oxides,perovskites, other organic salts, organic molecules, or polymers. Activelayers can be composed of neat planar layers of donor-acceptor pairs,mixed layers of blended donor-acceptor pairs, or graded layers ofblended donor-acceptor pairs. In various embodiments, the photo-activedevice 10, 10′ is integrated into a multijunction device architecture asa subcell, wherein the multijunction device is either visiblytransparent or visibly opaque. As described above, the photo-activedevice can be a photovoltaic or a photodetector.

Organic salts with deeper selective harvesting in the near-infrared witha range of selective-near-infrared peaks from about 700 nm to about 1600nm (FIG. 12A) further enable development of efficient and low-costmultijunction devices, both opaque and transparent, with complimentaryresponse across the solar spectrum (FIG. 12B). These organic salts alsoenable precise tuning of frontier orbital levels and heterojunctioninterface gaps through anion alloying, i.e., blending two or more anionstogether, that result in voltages near the thermodynamic limit (FIG. 13)that can provide an independent tuning parameter to optimize the productof photocurrent and photovoltage.

Accordingly, the present technology also provides a method for adjustinga frontier energy level, or highest occupied molecular orbital (HOMO),position of a material in a photo-active device. The method includesblending two or more organic salts together, wherein the organic saltshave the same cations, but different anions. Blending the organic saltstogether generates an anion alloy, i.e., a composition comprising ahomogeneous cation and a plurality of anions. For example, an anionalloy generated from two different organic salts having a common cationis a two-anion one-cation mixture. The anion alloy may be in the form ofa thin film. The method also comprises disposing the anion alloy betweena first electrode and a second electrode. One of the first electrode orthe second electrode is positioned directly or indirectly on asubstrate. The anions and cations can be any anions and cationsdescribed herein. The different organic salts blended together may beblended together in equimolar amounts or in non-equimolar amounts. Byadjusting energy level positions, custom photo-active devices may bemanufactured. In various embodiments, the method also includes disposingadditional layers to the photo-active device as discussed above.

Embodiments of the present technology are further illustrated throughthe following non-limiting examples.

Example 1 Experimental

Materials and Synthesis:2-[2-[2-chloro-3-[2-(1,3-dihydro-3,3-dimethyl-1-ethyl-2H-benz[e]indol-2-ylidene)ethylidene]-1-cylohexen-1-yl]-ethenyl]-3,3-dimethyl-1-ethyl-1H-benz[e]indoliumiodide (Cyl) was purchased from the American Dye Source (Quebec, Canada)and was filtered through a plug of silica with a 5:1 DCM:MeOH solventmixture before use. Lithium tetrakis(pentafluorophenyl)borate ethyletherate, NaPF₆ (98%), and AgSbF₆ (98%) were purchased from SigmaAldrich (St. Louis, Mo.) and used as received. Δ-TRISPHATtetrabutylammonium salt (98.5%) was purchased from Santa CruzBiotechnology (Dallas, Tex.), C₆₀ (99.9%) was purchased from MER(Materials and Electrochemical Research Corp, Tucson, Ariz.),bathocuproine (BCP) was purchased from Luminescence Technology Corp.(Hsin Chu, Taiwan), and MoO₃ (99.9995%) was purchased from Alfa Aesar(Ward Hill, Mass.). All exchanges followed procedures known in the art,except for CySbF₆, which was exchanged via a precipitation reaction withAgSbF₆ in dichloromethane (DCM). Filtering all exchange products througha plug of silica with DCM as an eluent allowed for the more polarremnant Cyl to be easily removed.

Device Fabrication and Testing: Glass substrates pre-patterned with 1200Å of ITO were cleaned by sequential sonication in soap, deionized water,and acetone for four minutes each, followed by boiling in isopropanolfor five minutes and oxygen plasma treatment for five minutes. Alldevices were fabricated with an active area of 4.9 mm². Compounds(1)-(4) of FIGS. 2A and 2B were spin-coated for 5 s at 600 rpm followedby 20 s at 2000 rpm from chlorobenzene solutions of varyingconcentrations. Compound (5) of FIGS. 2A and 2B was dissolved in a 3:1CB:DCM solution due to limited solubility in CB. All other layers weredeposited at 1 Å/s via vacuum thermal evaporation with a pressure below3·10⁻⁶ torr. The device structure for opaque devices was: MoO₃ (100Å)/CyX (x Å)/C₆₀ (400 Å)/BCP (75 Å)/Ag (800 Å). The device structure fortransparent devices was: MoO₃ (100 Λ)/CyX (x Å)/C₆₀ (400 Å)/BCP (75Å)/Ag (20 Å)/MoO₃ (1000 Å)/ITO (1000 Å) as shown in FIG. 2C. Filmthicknesses and optical constants were determined using ellipsometry.Current density (J) was measured as a function of voltage (V) under darkconditions and AM1.5G solar simulation (xenon arc lamp) where theintensity was measured using a NREL-calibrated Si reference cell withKG5 filter. J-V characteristics were averaged over at least 24 devicesfabricated in three independent device sets. External quantum efficiency(EQE) measurements were calibrated using a Newport calibrated Sidetector.

Optical characterization: Specular transmittance of films and devicestacks were measured using a dual-beam Lambda 800 UV/VIS spectrometer inthe transmission mode without the use of a reference sample.

Optical Modeling: Exciton diffusion lengths were fit from EQE data usingtransfer matrix optical modeling. This modeling was also used toidentify optical layer thickness structures for both opaque andtransparent devices.

Results and Discussion

The synthesis, characterization, and photovoltaic device performance ofa series of organic salts with response past 800 nm and up to 1600 nmwas realized. A key core NIR active heptamethine cations (Cy⁺) was usedas an exemplary organic salt to explore the impact of varying counterionsubstitutions (See FIGS. 2A and 2B). To improve the accuracy ofcomparisons between counterions, each salt was prepared by a single-stepsolution-based counterion exchange from a single batch of Cyl. Thesecounterions were chosen to illustrate the effects of counterion size,electronegativity, and molecular structure on solid state photophysicalproperties and device performance.

FIG. 3A shows J-V characteristics of the most representative devices foreach counterion under one sun illumination and FIG. 3B shows J-Vcharacteristics in dark conditions. It is immediately apparent that thecounterion plays a dramatic role in J-V performance. Cyl and CyBF₄ showlow fill factors (0.3-0.4), whereas CyPF₆, CySbF₆, CyTRIS, and CyTPFBshow fill factors as high as 0.64. BF₄ ⁻ and I⁻ are also the smallest,most mobile counterions. Table 1 provides J-v characteristics for eachcounterion.

TABLE 1 J-v characteristics for each counterion J_(SC) V_(OC) FF PCEDevice [mA cm⁻²] [V] [—] [%] Transparent 2.6 0.69 0.53 0.95 TPFB OpaqueTPFB 5.57 0.71 0.60 2.34 Δ-TRISPHAT 5.06 0.63 0.67 1.97 SbF₆ ⁻ 4.85 0,450.64 1.40 PF₆ ⁻ 4.73 0.41 0.63 1.24 I⁻ 4.79 0.45 0.32 0.70

Normalized extinction coefficients of each salt are shown in FIG. 4A andexternal quantum efficiencies (EQEs) of the devices are shown in FIG.4B. Transmission scans of transparent devices are shown in FIG. 5A andFIG. 5B shows a transparent device placed over a Michigan StateUniversity Spartan helmet logo to demonstrate transparency. Device datafor CyTFMPB is shown in FIGS. 15A and 15B.

Intensity dependent J-V parameters for each counterion are shown inFIGS. 6A-6D. As shown in FIG. 6B, CySbF₆ showed minimal Voc enhancementof ˜0.03 V over CyPF₆, with a comparable Voc as Cyl. However, much moresubstantial Voc increases were observed with the Δ-TRISPHAT and TPFBanions (0.2 V and 0.3 V greater than the Voc of CyPF₆, respectively).

The most interesting feature observed in the dark curves is in theforward bias. Counterions showing a higher Voc show a correspondingincrease in ideality factor. Despite the dramatic increase in Voc forCyTPFB devices, there is no appreciable decrease in the reversesaturation dark current, Js. Voc enhancements are commonly understood bya reduced leakage current, but this mechanism does not appear to besignificant in this case.

To characterize exciton diffusion length, EQE was measured as a functionof thickness for each counterion. The results are provided in FIGS.7A-7D. CyTPFB and CyTRIS show significantly less EQE roll-off withincreasing thickness, confirming an enhancement in exciton diffusionlength, charge collection length, or both. This trend can also be seenin FIG. 8, where the peak EQE is plotted as a function of thickness foreach anion and follows the trend of fitted Lo enhancement. In reality,the charge collection efficiency, η_(CC), could diminish at largerthicknesses, thus suggesting that the diffusion lengths are likely lowerestimates.

To investigate whether mobile ion shunting and the development of aspace charge region was significant in these devices, we performedpoling experiments by biasing the devices under −1 V for 30 min. Voc andFF did not increase appreciably in CyPF₆ or CyTPFB devices.

When studying single crystals it has been found that TPFB shows a highersymmetry environment surrounding the cation in single crystals. However,the films incorporated into the current devices have shown nocrystallinity and are completely amorphous.

FIG. 4A shows the normalized extinction coefficients for each counterionas measured by ellipsometry. CyTPFB and CyTRIS show slightly narrowerabsorption widths compared to the other counterions, with absorptionranges of approximately 650-930 nm and 670-930 nm, respectively,compared to 620-950 nm for Cyl, CyPF₆ and CySbF₆. This effect can likelybe attributed to a combination of the increased separation (andcorresponding reduced aggregation and interaction) between Cy cationsand the decreased polarization of the Cy cation seen in crystallinesolids of these salts. Both of these effects would result in a decreaseddistribution of available states for absorption. While the narrowing ofthe peaks does correspond to a slight decrease in the optical gap of˜0.03 eV, this is clearly insufficient to explain a 0.3 V increase inVoc.

Peak narrowing has the advantage of increasing average visibletransmittance and color rendering index for transparent photovoltaicapplications. The long wavelength cut-off of photopic response is at−675 nm, and as can be seen in FIG. 4A, the CyTPFB and CyTRIS haveconsiderably less overlap with the photopic response curve than theother anions, Cy(1)-(4). Narrowing could also have the benefit ofdecreasing competitive absorption with ClAlPc or other NIR donors inmultijunction transparent photovoltaics for enhanced performance.

This work presents molecular design strategies that enable facile tuningof energy level alignment and open-circuit voltages in organicsalt-based photovoltaics via anion alloying or blending. With increasingCyTPFB content in a CyPF₆-CyTPFB mixture, a steady linear increase inVoc (see FIGS. 15A and 15B) and a significant decline in Jo, isobserved, which follows the expected behavior for interface gapmodulation and HOMO level shifts. Similar linear behavior for mixingwith CySbF₆ and CyTRIS, as well as with other cation/anion combinationsare also observed. Thus, this identifies the key mechanism responsiblefor the voltage enhancement and is the first clear evidence in supportof an energy level alignment explanation in such systems. This mechanismprovides a powerful new tuning parameter for optimizing the highestoccupied molecular orbital (HOMO) position to balance charge generationvia charge transfer efficiency with interface recombination dynamics.

CONCLUSION

Open circuit voltage enhancement from 0.40 V to 0.72 V via counterionexchange in organic photovoltaics with photocurrent generation past 800nm and up to 1600 nm has been demonstrated. Relative to the opticalexcitonic gap of this molecule, this is the highest reported Voc for acyanine-based photovoltaic, nearing the excitonic limit. This increaseto the elimination of mid-gap defect states and ion mobility isattributed to the steric hindrance of the aromatic counterions.Moreover, these new molecules show exceptional potential for transparentphotovoltaics through selective near-infrared harvesting. This workdemonstrates how to achieve high voltage near-infrared absorbing organicphotovoltaics and highly efficient transparent photovoltaics.

Example 2 Introduction

Few organic photovoltaics (OPVs) and organic photodetectors (OPDs) havedemonstrated a photoresponse past 900 nm, a previously under-utilizedspectral region for tandem solar cells, transparent solar cells andinfrared photodetectors. Here, heptamethine salts with selective deepnear-infrared (NIR) photoresponses are demonstrated with externalquantum efficiencies (EQEs) cutoffs at λ=1400 nm or 1600 nm. Anionexchange is shown to deepen frontier orbital levels with minimal changesin absorption properties, leading to decreases in dark current,increases in open-circuit voltage (approaching excitonic limits), andincreases in specific detectivity. Balancing exciton binding energy andcharge transfer efficiency is shown to be key for enhancing theperformance of very small bandgap NIR-absorbing devices. These organicsalts represent a pathway to inexpensive infrared solar cells anddetectors, expanding the catalog of existing donor materials fortransparent and multijunction solar cells.

Organic semiconductors that absorb in the NIR, i.e., at λ>800 nm, arepromising for applications in broadband and transparent solar cells.Organic compounds with NIR photovoltaic response have been demonstratedincluding cyanines, carbon nanotubes, and polymers. However, EQEs inthese studies have only extended to 1100 nm for SnNcCl₂ and 1450 nm forcarbon nanotubes. Design strategies for redshifting the IR absorption oforganic molecules have included increasing the conjugation and modifyingthe ligand structures to affect aggregation, crystal structure, andintermolecular proximities. However, once molecules are designed andintegrated into optoelectronic devices, their performance typicallysuffers from arbitrary energy level alignments, resulting inlower-than-ideal open-circuit voltages, low carrier mobilities anddiffusion lengths, and limited absorbance past 1000 nm. A new series ofheptamethine salts with highest occupied molecular orbital (HOMO) levelsthat can be tuned by varying the anion electronegativity are nowprovided. These organic salts are used in photovoltaic and photodetectorcells to demonstrate photoresponse at deep NIR wavelengths andopen-circuit voltages nearing their excitonic limit. Using opticalmodeling and open-circuit voltage tuning, limiting factors forperformance and strategies for performance enhancement are identified.

Heptamethine salts 1(1-Butyl-2-(2-[3-[2-(1-butyl-1H-benzo[cd]indol-2-ylidene)-ethylidene]-2-diphenylamino-cyclopent-1-enyl]-vinyl)-benzo[cd]indolium,λ_(max)=996 nm) and 2(1-Butyl-2-(2-[3-[2-(1-butyl-1H-benzo[cd]indol-2-ylidene)-ethylidene]-2-phenyl-cyclopent-1-enyl]-vinyl)-benzo[cd]indolium,λ_(max)=1024 nm) coordinated with the counterions tetrafluoroborate (BF₄⁻) and tetrakis(pentafluorophenyl)borate (TPFB⁻) are shown in FIG. 16A.As shown in FIG. 16B, these molecules have absorption ranges that extendto 1400 nm and 1600 nm for cations 1 and 2 respectively. FIG. 16C showsa summary of the m/z synthesis verification for the cation and anionmasses.

Experimental

Synthesis of 1-TPFB and 2-TPFB: Equimolar amounts of potassiumtetrakis(pentaflurophenyl)borate (K-TPFB) and either 1- or 2-BF₄ weredissolved in 5:1 methanol:dichloromethane (MeOH:DCM) at 10 mg/ml andstirred at room temperature under nitrogen for 1 hr prior to reaction.All chemicals were used as received (Boulder Scientific Company, Few),and solvents were HPLC grade (Sigma Aldrich). The solid product wascollected using vacuum filtration and an MeOH wash, redissolved inminimal DCM (˜10 mg/ml), and poured through a plug of silica using DCMas eluent to remove impurities and unreacted 1- or 2-BF₄. The firstfraction with similar color to 1- or 2-BF₄ was collected, and excess DCMwas removed in a rotary evaporator at 55° C. for 20 min at atmosphericpressure.

Verification of 1-TPFB and 2-TPFB and ion purity assessment:Verification of cations, anions, and product purity were performed usinga Waters Xevo G2-XS QToF mass spectrometer coupled to a Waters Acquityultra-high pressure LC system. Cations were analyzed in positive ionmode electrospray ionization (ESI), and anions were analyzed in negativeion mode ESI. Solutions were prepared in acetonitrile and directlyinjected for 2 minutes using an eluent of 50:50 water:acetonitrile. Massspectra were acquired using a dynamic range extension over m/z 50 to1,500, with mass resolution (M/ΔM, full width-half maximum) ofapproximately 20,000. Other parameters include capillary voltage of 2kV, desolvation temperature of 350° C., source temperature of 100° C.,cone gas (N₂) at 0 L h⁻¹, and desolvation gas (N₂) at 400 L h⁻¹. The m/zcalculated for cation 1 [C₅₁H₄₈N₃]⁺ is 702.3848, and the measured m/z is702.3641. The m/z calculated for TPFB⁻ anion [C₂₄BF₂₀]⁻ is 678.9774, andthe measured m/z is 678.9788. The m/z calculated for cation 2[C₄₅H₄₃N₂]⁺ is 611.3426, and the measured m/z is 611.3421. The m/zcalculated for TPFB⁻ anion [C₂₄BF₂₀]⁻ is 678.9774, and the measured m/zis 678.9789.

For ion purity assessment, solutions of the exchange precursors andproducts were prepared in acetonitrile with concentrations varying from10 nM to 500 nM and analyzed by mass spectrometry as described above.Calibration curves of integrated ion detection intensity for precursorsK-TPFB, 1-BF₄ and 2-BF₄ vs. concentration were calculated to measure BF₄and TPFB⁻ ion concentrations in the exchange products 1-TPFB and 2-TPFB.The ion purity was measured to be >95% TPFB for both 1-TPFB and 2-TPFB.

Solar cell device fabrication and testing. Patterned ITO substrates (XinYan, 100 nm, 20 Ω/sq) were sequentially cleaned in soap, DI water,acetone and boiling isopropanol for three minutes each. Substrates werethen oxygen plasma treated for three minutes, and MoO₃ (99.9995%, AlfaAesar) was thermally evaporated at 0.1 nm/s at 3×10⁻⁶ torr. Heptamethinesalts were massed in air, dissolved in dimethyl formamide under anitrogen environment, and sonicated for at least thirty minutes.Solutions were used without filtering and spincoated at 3000 rpm for 30sec in a glovebox. Subsequent layers of C₆₀ (99.9%, MER Corp.),bathocuproine (BCP, Luminescience Technology, Inc.) and silver werethermally evaporated at 0.1, 0.05, and 0.2 nm/s, respectively. Layerthicknesses were measured using variable-angle spectroscopicellipsometry (J. A. Woollam) on Si substrates. Device areas (averagevalue: 5.7 mm²) were defined as the area of overlap between the anodeand cathode and were measured using optical microscopy. Current density(J) was measured as a function of voltage using a Labview-controlledsourcemeter under xenon arc lamp illumination calibrated for AM1.5G (100mW/mm²) intensity using a NREL-calibrated Si reference cell with KG5filter. External quantum efficiency (EQE) measurements were performed byusing monochromatic light from a tungsten halogen lamp chopped at 200Hz, a picoammeter and a lock-in amplifier. The light intensity at theend of the IR-fiber was measured using a Newport calibrated Si diode for350-800 nm and a Newport calibrated Ge diode for 800-1600 nm. Specificdetectivity D*(cm Hz^(1/2) W⁻¹) was calculated based on measurements atshort circuit (V=0). D* is obtained from:

D*=R√{square root over (A)}S _(N) ⁻¹  (1)

where R is responsivity in A/W, A is device area in cm², and S_(N) ⁻¹ iscurrent spectral noise density in A Hz^(−1/2). At room temperature and0V, the noise is dominated by thermal (Johnson-Nyquist) noise S_(T) (AHz^(−1/2)), which is estimated as:

$\begin{matrix}{S_{T} = \sqrt{\frac{4k_{B}T}{R_{D}}}} & (2)\end{matrix}$

where k_(B) is the Boltzmann constant (J K⁻¹), T is temperature (K), andR_(D) is the differential resistance of a solar cell in the dark at zerobias.

Ultraviolet photoelectron spectroscopy data were recorded with a He lampemitting at 21.2 eV (He I radiation) on ˜10 nm thick salt films onMoO₃/ITO. The samples were loaded without exposure to air. LUMOtransport levels were estimated by adding the optical bandgaps (0.85 eVfor 1-BF₄ and 1-TPFB and 0.80 eV for 2-BF₄ and 2-TPFB) and calculatedexciton binding energies (0.55 eV for 1-BF₄ and 1-TPFB and 0.40 eV for2-BF₄ and 2-TPFB) to the HOMO levels.

Results and Discussion

As shown in FIG. 17A, solar cell devices with the structure indium tinoxide (ITO)/10 nm MoO₃/t nm salt/40 nm C₆₀/7.5 nm bathocuproine (BCP)/80nm Ag were prepared using the four salts as a function of thickness.Donor layers of each organic salt were spin-coated fromN,N-dimethylformamide (DMF) under nitrogen while other layers werethermally deposited in vacuum. The thickness for each salt wascontrolled by varying the solution concentration. For comparisonpurposes, the J-V and EQE for devices with similar salt thicknesses(12±1 nm) are plotted in FIGS. 17B and 17C and average performancemetrics are shown in Table 2. The exchange of BF₄ for TPFB nearlydoubles the V_(OC) from 0.13 to 0.33 V for cation 1 and 0.17 to 0.25 Vfor cation 2. This enhancement in the voltage is due to the shift infrontier energy levels and increased interface gap as shown in FIG. 18A.However, this exchange reduces the NIR EQE peak by more than 50% due toa substantial decrease in the donor-acceptor lowest unoccupied molecularorbital level offset (Δ_(LUMO)). To Understand the Effect of GradualShifts in Interface gap on EQE, alloyed blends of 1 or 2-BF₄ withvarying molar ratios of 1- or 2-TPFB were prepared. The V_(OC) and EQEtrends as a function of TPFB molar fraction are plotted in FIG. 19A.

TABLE 2 Device parameters and molecular properties for each salt. NIRJ_(SC) Abs. EQE D* (mA V_(OC) Edge Peak Peak J₀ Salt cm⁻²) (V) FF (nm)(%) (Jones) (μA/cm²) 1-BF₄ 3.4 ± 0.13 ± 0.34 ± 1440 2.1 3.7 × 48 0.30.01 0.01 10⁹ 1-TPFB 1.9 ± 0.33 ± 0.49 ± 1460 1.1 5.3 × 0.014 0.2 0.010.01 10¹⁰ 2-BF₄ 3.4 ± 0.15 ± 0.42 ± 1590 1.4 7.0 × 7.0 0.3 0.01 0.05 10⁹2-TPFB 1.7 ± 0.25 ± 0.42 ± 1500 0.8 1.7 × 1.1 0.2 0.01 0.04 10¹⁰

The thickness trends of the pure salts are plotted in FIG. 19B, whereV_(OC) either remains flat (1-BF₄ and 2-BF₄) or decreases (1-TPFB and2-TPFB) with increasing thickness and EQE monotonically increases. Ingeneral, the V_(OC) is found to be independent of donor salt thicknessin the 5-15 nm range. In some cases, open-circuit voltages for OPVsincrease with thickness as parallel shunting pathways are eliminated bythe formation of more complete films. In the case of 1- and 2-TPFB,however, V_(OC) shows a modest decrease of 20% over the thickness rangeof 4 to 15 nm. Decreases in V_(OC) with increasing thickness have beenattributed to (1) increased recombination due to presence ofdisorder-induced gap tail states, (2) increased recombination due toelectric field profile broadening and (3) shifts in the interface gapdue to band bending (as shown in FIG. 18B). Mechanisms (1) and (2) areunlikely due to the small thickness range (1 nm) over which the voltagedrop occurs; thus, the V_(OC) decrease is most likely due to incompleteband bending in 1- and 2-TPFB devices as a function of thickness. Incontrast, devices with 1- and 2-BF₄ have no thickness dependentphotovoltage and therefore likely have smaller depletion widths stemmingfrom either larger carrier densities or smaller dielectric constants.

Quantum efficiencies past 1000 nm have been limited in magnitude to <15%even for many quantum dot systems. To identify the limiting factors inthe current NIR EQE, component efficiencies were examined. EQE can beexpressed as the product of the internal efficiencies: η_(A)(absorption), η_(ED) (exciton diffusion), η_(CT) (charge transfer),η_(CD) (charge dissociation) and η_(CC) (charge collection). Throughexciton diffusion and optical interference modeling, EQE curves werewell fit for effective exciton diffusion lengths, which were calculatedassuming 100% charge transfer, charge dissociation, and chargecollection efficiencies. From this analysis it was found that theeffective diffusion lengths in these four salts are all from about 0.5nm to about 1 nm due to the modest EQEs. However, it was also found thatabsorption profiles already reach 70% at the peak wavelength for 2-TPFBfilms that are only 25 nm thick, suggesting that these devices are notlimited by absorption. With the extracted diffusion lengths, the opticalinterference model predicts that the EQE should decrease for all thetested salts with increasing thickness due to the inability of excitonsto diffuse to the dissociating interface. This predicted trend ofdecreasing EQE is indeed seen experimentally in other larger gap cyaninesalt devices. However, this behavior is in contrast to the experimentaltrends here, which show EQE monotonically increasing for donor layerthicknesses past 25 nm. This suggests that the intrinsic diffusionlength is in fact longer than 0.5-1 nm and the EQE of these salts isinstead limited by charge transfer, charge dissociation, or chargecollection efficiency, at least one of which should not be modeled as100%.

While there is not a clear method to directly distinguish between all ofthese component efficiencies (charge transfer, dissociation, andcollection), insight about charge collection from other measurements canbe inferred. For example, since the experimental C₆₀ EQE peak (Λ=430 nm)does not decrease with increasing salt thickness and is similar inmagnitude to other salt based OPVs with larger bandgaps, this impliesthat hole collection from excitons originating on C₆₀ (which still haveto transport through the donor salt) is not a limiting factor. Thus,devices are most likely limited by charge transfer or dissociationefficiency as a result of the balance between the lowest unoccupiedmolecular orbitals of the donor and acceptor and the exciton bindingenergy.

To estimate the exciton binding energy, anion mixing experiments wereperformed and the emergence of sharp cutoffs in the EQE (FIG. 19A) weresought. Indeed, while there is a linear variation in the V_(OC) thatstems from a monotonic modulation of interface gap recombination, thereis a sharp EQE cutoff at a molar fraction of 10% TPFB, suggesting thatthere is just enough energy (Δ_(LUMO)) available at that concentrationto efficiently overcome the exciton binding energy. The remainingquantum efficiency beyond this concentration likely stems from acombination of field- and temperature-driven dissociation. The energyavailable for exciton dissociation was estimated by subtracting theinterface gap (calculated from the V_(OC)) from the optical bandgap,yielding exciton binding energies of about 0.55 eV for 1 and about 0.40eV for 2, which is close to other reported values in organic molecules(from 0.2 to 1.4 eV). These exciton binding energies make up roughly 50%of the optical bandgap (about 0.8 eV), limiting the interface gap (andtherefore V_(OC)) to modest values at which efficient excitondissociation can still take place, despite the ability to achieve higherV_(OC)s. Moving forward, several strategies can be explored to decreasethe exciton binding energy. For example, molecular modifications can bedesigned to enhance the delocalization of the electron/hole orbitals toincrease the exciton radius, e.g., via central methine substitution.Another design strategy involves the coupling of smaller solubilizinggroups or anions that allow for denser packing to increase thedielectric constant. Thus, this presents an interesting design challengefor the future optimization of very small bandgap organic photoactivedevices.

To understand the ultimate potential of these material sets inphotovoltaic applications, the EQE of a device with a 100 nm thick2-TPFB layer having an exciton diffusion length of 100 nm, a 20 nm thickC60 layer, and charge transfer and charge collection efficienciesapproaching 100% was modeled. Such a device would have an EQE of about70-80% with a J_(SC) on the order of 25 mA/cm², and could be realizedusing a bulk heterojunction architecture and optimized energy leveltuning. FF values can be increased from about 0.3 to about 0.65(achievable for many organic systems) by optimizing the interfaceenergetics or the modification of solvent processing conditions.Combined with a slightly improved V_(OC) of 0.55 V, which is around theShockley-Queisser excitonic limit, ideal devices would be 10% efficientwith high transparency and would be well suited for multijunction cellswith complimentary absorption.

These salt-based devices are also shown to be viable for near infraredphotodetectors. Photodetector devices were fabricated with the samephotovoltaic structure as those above. Specific detectivity (D*) curvesfor each salt are plotted in FIG. 20, where D* is proportional to theEQE and inversely proportional to the differential resistance at zerobias. Calculated D*s are comparable to those of the limited reports inother organic systems and reach values of 6.5×10¹⁰ Jones at λ=1140 nmand 1.7×10¹⁰ Jones at λ=1390 nm for 1- and 2-salts respectively. Theseobservations compare well to the very limited reports in organicsystems, such as porphyrin tapes (2.3×10¹⁰ Jones at λ=1350 nm) andinorganic carbon nanotubes (8.8×10¹¹ Jones at λ=1350 nm). Exchanging theanion from 1-BF₄ to 1-TPFB increases the detectivity by an order ofmagnitude from 3.7×10⁹ to 5.3×10¹⁰ Jones, largely due to lower noisecurrents for devices with the TPFB anion. Compared to other organicsystems, heptamethine salts have readily tunable properties viacounterion or ligand exchange in addition to being easy to synthesizeand fabricate. Moreover, they exhibit both broader andwavelength-specific photoresponsivity, which is promising for a range ofapplications in the near-infrared and visibly transparentphotodetectors.

In summary, simple organic salts with unusually low bandgaps (0.8 eV)for infrared photoresponsivity extending to 1600 nm have beendemonstrated. These salts are demonstrated in both photovoltaics andphotodetectors and obtain peak NIR EQEs approaching 5% with standardfullerene acceptors. Performing counterion exchanges on theseheptamethine salts is shown to increase the interface gap—along withV_(OC), dark saturation current, and D*—with an eventual tradeoff in theexciton dissociation and quantum efficiency due to the modest excitonbinding energies (from about 0.4 to about 0.55 eV). Nonetheless, anionexchange and alloying allow for facile tuning of the interface gap andprovide interesting insight into the binding energies of these verysmall bandgap salts. These heptamethine salts represent a new approachto extend the range of NIR photoresponsive devices and enable new routesto the development of low cost infrared detectors and high efficiencymultijunction cells.

The foregoing description of the embodiments has been provided forpurposes of illustration and description. It is not intended to beexhaustive or to limit the disclosure. Individual elements or featuresof a particular embodiment are generally not limited to that particularembodiment, but, where applicable, are interchangeable and can be usedin a selected embodiment, even if not specifically shown or described.The same may also be varied in many ways. Such variations are not to beregarded as a departure from the disclosure, and all such modificationsare intended to be included within the scope of the disclosure.

What is claimed is:
 1. A photovoltaic device comprising: a firstelectrode; a second electrode; and an active layer comprising an organicsemiconductor positioned between the first electrode and the secondelectrode, wherein the photovoltaic device has an open circuit voltagethat is within 30% of the excitonic limit.
 2. The photovoltaic deviceaccording to claim 1, wherein the organic semiconductor selectively orpredominantly absorbs light in at least one of the near infrared orinfrared regions of the solar spectrum.
 3. The photovoltaic deviceaccording to claim 1, wherein the organic semiconductor is an organicsalt.
 4. The photovoltaic device according to claim 3, wherein theorganic salt absorbs light having a wavelength from about 675 nm toabout 1600 nm.
 5. The photovoltaic device according to claim 4, whereinthe photovoltaic device is visibly transparent.
 6. The photovoltaicdevice according to claim 5, having an average visible transparency,weighted by the photopic response of an eye, of about 50% or more forspecular transmission.
 7. The photovoltaic device according to claim 3,wherein the organic salt comprises a counterion selected from the groupconsisting of halides, aryl borates, carboranes, phosphates,fluoroantimonates, fluoroborates, derivatives thereof, and combinationsthereof.
 8. The photovoltaic device according to claim 7, wherein theorganic salt comprises a polymethine ion or a cyanine ion.
 9. Thephotovoltaic device according to claim 3, wherein the active layercomprises a first organic salt and a second organic salt, wherein thefirst and second organic salts have a common cation, but differentanions.
 10. The photovoltaic device according to claim 1, wherein thephotovoltaic device is visibly opaque.
 11. The photovoltaic deviceaccording to claim 1, further comprising a complementary layercomprising an electron donor or an electron acceptor.
 12. Thephotovoltaic device according to claim 1, wherein the photovoltaicdevice is integrated in a multijunction device as a subcell.
 13. Thephoto-active device according to claim 1, wherein the open circuitvoltage is within 20% of the excitonic limit.
 14. A photovoltaic devicecomprising: a first electrode; a second electrode; and an active layerpositioned between the first electrode and the second electrode, theactive layer comprising: a first organic salt having an ion and a firstcounterion; and a second organic salt having the same ion as the firstorganic salt and a second counterion, the second counterion beingdifferent from the first counterion.
 15. The photovoltaic deviceaccording to claim 14, wherein the first and second organic saltsselectively or predominantly absorb light in at least one of the nearinfrared or infrared regions of the solar spectrum, and wherein thephotovoltaic device is visibly transparent, having an average visibletransparency, weighted by the photopic response of an eye, of about 50%or more for specular transmission.
 16. The photovoltaic device accordingto claim 14, wherein the first and second counterions are independentlyselected from the group consisting of halides, aryl borates, carboranes,phosphates, fluoroantimonates, fluoroborates, derivatives thereof, andcombinations thereof.
 17. The photovoltaic device according to claim 16,wherein at least one of the first or second counterions is not afluorophosphate.
 18. The photovoltaic device according to claim 14,having an open circuit voltage that is within 30% of the excitoniclimit.
 19. The photovoltaic device according to claim 18, wherein theopen circuit voltage is within 20% of the excitonic limit.
 20. A methodof manufacturing a photo-active device, the method comprising: disposingan active layer between a first electrode and a second electrode,wherein the active layer comprises an organic salt comprising acounterion selected from the group consisting of halides, aryl borates,carboranes, phosphates, fluoroantimonates, fluoroborates, derivativesthereof, and combinations thereof, wherein the phosphates do not includefluorophosphates, and wherein the active layer harvests light.